P. S. Wijewarnasuriya

The doping of mercury cadmium telluride grown by molecular‐beam epitaxy

The electrical properties of the mercury cadmium telluride semiconductor material grown by the molecular‐beam epitaxy are reviewed. The doping effects linked to the growth conditions, as well as the influence of indium or lithium incorporation are discussed. The results on doping by silicon, arsenic, and antimony are presented. It will be shown that all the impurities studied interact primarily with the metal site. It will confirm that the growth occurs under very rich tellurium conditions.

Arsenic incorporation in HgCdTe grown by molecular beam epitaxy

We report the results of in situ arsenic doping in HgCdTe layers grown by molecular beam epitaxy (MBE). Arsenic incorporation was carried out by two mechanisms called conventional doping and planar doping. The obtained results indicate that for both mechanisms, after Hg anneal, arsenic was successfully incorporated as an acceptor during the MBE growth. Secondary ion mass spectrometry and Hall-effect measurements before and after Hg annealing were used to characterize arsenic activity in the grown layers. Close to 100% acceptor doping efficiency with arsenic has been obtained on these MBE grown layers up to total arsenic concentrations of approximately 2×1018 cm−3, which is more than sufficient for a wide range of infrared devices. At much higher total arsenic concentrations, electrical activity falls off drastically as the doping level saturates.

Electrical properties of Li‐doped Hg1−xCdxTe(100) by molecular beam epitaxy

p‐type doping of HgCdTe(100) layers with lithium during growth by molecular beam epitaxy is reported. Hall measurements have been performed on these layers between 300 and 30 K. The Li concentration is found to increase with the Li cell temperature. Li‐doped HgCdTe layers are estimated to have very shallow acceptor levels. Acceptor concentrations as high as 8×1018 cm−3 have been achieved. At low doping levels, due to residual donors, layers show compensation. Incorporation coefficient of Li close to 1 and almost 100% electrical efficiency for Li in molecular beam epitaxy HgCdTe layers were observed. However, Li is found to diffuse rapidly in HgCdTe layers grown by molecular beam epitaxy.

Molecular beam epitaxial growth and characterization of Cd1−xZnxTe, Hg1−xCdxTe, Hg1−xMnxTe, and Hg1−xZnxTe on GaAs(100)

The growth of Cd1−xZnxTe on GaAs (100) is reported here for the first time. In contrast to CdTe, when x is higher than 0.15 only the Cd1−xZnxTe(100)∥GaAs(100) orientation is observed. We also present the growth and characterization of n‐type and p‐type layers of Hg1−xCdxTe, Hg1−xMnxTe, and Hg1−xZnxTe for the first time, in most of the cases. These layers exhibit high structural and good electrical properties. Several different characterizations carried out on Hg1−xCdxTe layers grown by MBE on GaAs substrates show that this material is at least as good as the best material grown by any other technique and confirm that GaAs is emerging as the candidate for replacement of CdTe as the substrate for growth of Hg1−xCdxTe by MBE. Hg1−xMnxTe and mainly Hg1−xZnxTe, which have electrical characteristics similar to high quality Hg1−xCdxTe, appear as potential candidates for replacement of Hg1−xCdxTe.

Carrier recombination in indium‐doped HgCdTe(211)B epitaxial layers grown by molecular beam epitaxy

We report the recombination mechanisms of minority carrier lifetime in indium‐doped layers of (211)B Hg1−xCdxTe(x ≊ 23.0% ± 2.0%)n‐type grown by molecular beam epitaxy. Measured lifetimes were explained by an Auger limited band‐to‐band recombination process in this material, even in the extrinsic temperature region. Frequently, in some of the layers, a combination of the band‐to‐band recombination mechanisms together with recombination at the Shockley–Read single level 33 to 45 meV below the conduction band was necessary to explain the measured data. Results indicate that these defects have acceptorlike characteristics and their origin is related to Hg vacancies.

Electrical properties of intrinsic p‐type shallow levels in HgCdTe grown by molecular‐beam epitaxy in the (111)B orientation

The electrical properties of the unintentionally doped p‐type HgCdTe material as grown in the (111)B orientation by molecular‐beam epitaxy are revised. The analysis of the Hall coefficient in the whole temperature range with a model based on the two‐band nonparabolic Kane model, a fully ionized compensating donor concentration, and two independent discrete acceptor levels is presented. The donor compensation is found to be much lower than before, in agreement with the latest study of extrinsic doping by indium. A defect level with an energy of 30 to 50 meV is found necessary to explain properly some of the crystals’ data. The results of a three‐carrier band modeling of the Hall constant versus field are also presented for one sample and are in very good agreement with the expected band structure of the material. These results show that important improvements have been made recently in the control of stoichiometry during growth.

Molecular beam epitaxial growth of a novel strained layer type III superlattice system: HgTe‐ZnTe

HgTe‐ZnTe strained type III superlattices have been grown for the first time using the molecular beam epitaxy technique. Three superlattices grown at 185 °C have been characterized by electron and x‐ray diffraction, infrared absorption, and Hall measurements. The presence of satellite peaks in the x‐ray spectra shows that the superlattices are of good quality despite the large lattice mismatch between HgTe and ZnTe (Δa/a=6.5%). These superlattices are p type and the hole mobilities are very high compared to those of the corresponding alloy. Such a phenomenon has already been reported for HgTe‐CdTe superlattices. Infrared transmission spectra show that HgTe‐ZnTe superlattices have narrower band gaps than equivalent HgZnTe alloys.

Evidence that arsenic is incorporated as As4 molecules in the molecular beam epitaxial growth of Hg1−xCdxTe:As

Molecular arsenic, As4, is commonly used as the source for in situ p-type doping of Hg1−xCdxTe grown by molecular beam epitaxy. As incorporated, the arsenic is strongly self-compensated, requiring annealing for its p-type electrical activation. Here, a quasithermodynamic model is used to interpret the dependence of the arsenic concentration, cAs, as measured by secondary ion mass spectroscopy, on the incident As4 and Hg fluxes during growth. The results strongly suggest that the As4 is absorbed in its molecular form rather than being dissociated on the growth surface, as has previously been assumed. This clearly is relevant to the self-compensation of the arsenic in as-grown Hg1−xCdxTe.

Annealing experiments in heavily arsenic-doped (Hg,Cd)Te

Arsenic doped molecular beam epitaxy (MBE) (Hg,Cd)Te films were grown on (Cd,Zn)Te substrates. The concentration of arsenic was varied from 5 x 1018 cm-3 to 1 x 1020 cm-3. After the growth, the epitaxial layers were annealed at various partial pressures of Hg within the existence region of (Hg,Cd)Te at temperatures ranging from 400 to 500°C. Hall effect and resistivity measurements were carried out subsequent to the anneals. 77K hole concentration measurements indicate that for concentrations of arsenic <1019 cm−3, most of the arsenic is electrically active acting as acceptors interstitially and/or occupying Te lattice sites at the highest Hg pressures. At lower Hg pressures, particularly at annealing temperatures of 450°C and higher, compensation by arsenic centers acting as donors appears to set in and the hole concentration decreases with decrease in Hg pressure. These results indicate the amphoteric behavior of arsenic and its similarity to the behavior of phosphorus in (Hg,Cd)Te previously inferred by us. A qualitative model which requires the presence of arsenic occupying both interstitial and Te lattice sites along with formation of pairs of arsenic centers is conjectured.

Electrical properties of molecular beam epitaxy produced HgCdTe layers doped during growth

Electrical characterizations of n‐ and p‐type mercury cadmium telluride epitaxial layers intentionally doped during their growth by the molecular beam epitaxy technique are reported. The doping by stoichiometry adjustment can produce good mobility material of both n or p semiconductor types. However, p‐type material is feasible only for cadmium fraction (x) greater than 0.2, whereas the n‐type conduction is possible only for x less than 0.3. For the first time in situ doping of HgCdTe by indium is reported. It is incorporated in the material during the growth and behaves as a n‐type dopant. Carrier concentrations up to 1018 cm−3 have been obtained. An indium doped layer having a cadmium fraction of 0.55 was also found to be n‐type.